1. Field of the Invention
This invention relates generally to thermal sensors of fluids, such as fluid flow or property sensors implemented on silicon in microstructure form. For convenience sake the term "flow sensor" will be used generically hereinafter for such thermal sensors. The reader will appreciate that such sensors may be utilized to measure primary properties such as temperature, thermal conductivity and specific heat; and that the flows may be generated through forced or natural convection. The invention relates more specifically to a sensor package of the microbridge or membrane type flow sensor having a central heating element and surrounding sensors which are capable of handling high pressure and have very low susceptibility to environmental damage or contamination.
2. Description of Related Art
Open microbridge structures such as detailed in U.S. Pat. No. 4,501,144, to Higashi, are well suited for measurements of clean gases, with or without large pressure fluctuations, since the microbridge structure is burst-proof. However, due to the open nature of the microbridge structure, condensates from vapor can be uncontrollably retained in the microbridge structure leading to uncontrolled changes in its thermal response, or output, making the structure susceptible to output error and poor stability. Also, in the typical microbridge structure, the silicon die is wire bonded at the top surface to a header, or substrate, carrying further electrical leads and/or electronics. Typically, such wire for the wire bonds would be a one mil gold wire. This wire has a further tendency to retain liquid condensates, increase undesirable turbulence, shift flow response. Due to its thinness, the wire is susceptible to damage in a high mass flux environment, such as liquid flow, and upon attempts to clean the sensor.
Membrane-based sensors such as shown in U.S. Pat. No. 5,705,745, to Treutler et al., overcome some of the problems of the microbridge structure because there is no opening between the bridge and the underlying thermal isolation cavity or air space. However, because the membrane is sealed over the isolation air space membrane based sensors have limited application in constant, near-atmospheric pressure, because the membrane can deform or burst as pressure differences increase above 100 PSI. The top surface of the membrane sensors is also typically wire bonded, leaving the problem of the wire in the flow path accumulating debris and possible breakage during cleaning attempts.
It would therefore be desirable to develop a flow sensor which is not susceptible to the above problems of vapor accumulation beneath the microbridge, poor ruggedness under high pressure capability of the membrane sensors, and exposed bonding wire near the heating and sensing elements. The design of such a structure would enable high pressure thermal property sensing over wide ranges at a reasonable cost and provide trouble free operation in heretofore hostile environments.